Process of preparing linear terephthalate polyester structures



United States Patent 3,091,510 PROCESS OF PREPARING LINEAR TEREPHTHAL- ATE POLYESTER STRUCTURES Eugene B. McCord and Paul T. Scott, Kinston, N.C., as-

signors to E. I. du Pont de Nemours and Company,

Wilmington, DeL, a corporation of Delaware No Drawing. Filed Mar. 16, 1962, Ser. No. 180,355 7 Claims. (Cl. 18-54) This invention relates to spontaneously extensible filaments and other shaped structures of synthetic linear polyesters. More particularly, it relates to a novel process for producing linear terephthala-te polyester filaments which have the property of undergoing a spontaneous and irreversible extension in length when they are heated.

The production of spontaneously extensible linear terephthalate polyester filaments has been described by Kitson and Reese in their United States Patent 2,952,897. In accordance with their disclosure, the starting material may he a filament which is spun or extruded from a molten linear terephthlate polyester and then drawn to orient it as described by Whinfield and Dickson in their United Staes Patent 2,465,319. An alternative starting material filament which may be employed is a filament oriented during the extrusion step by simply winding the extruded filament at a very high rate of speed, 3000 yards per minute or higher, as disclosed by Hebeler in his United States Patent 2,604,689. In either case, Kitson and Reese subject the starting material filament to a process in which -a key step involves shrinkage of the filament between about 20% and 70%, preferably 35% or more. While this shrinkage step has hitherto been regarded as essential in producing a spontaneously extensible linear terephthalate polyester filament, the fact that the process involves such severe shrinkage has been considered unfortunate, since in the manufacture of filaments a shrinkage step is inherently a step which operates to reduce productivity.

Experimental evaluation of processes described hitherto has also shown that the amount of shrinkage of the oriented filaments is always greater than the amount of growth obtained when the shrunk filaments are heated. Thus, although the shrunk filament is spontaneously extensible when heated, the filament is not as long after the step of spontaneous extension as it was just prior to the shrinkage step. In other words, the net change in length observed with respect to the original, oriented filament is shrinkage rather than extension. It is usually also observed not only that there is a net shrinkage, but also that the degree of shrinkage of the filament in the shrinkage step is much greater than the degree of growth of the filaments in the extension step.

It has now been found that certain oriented polyester filaments may be caused to become spontaneously and irreversibly extensible without the use of a shrinkage step. It has also been found, alternatively, that the filaments may be treated so that a subsequent shrinkage step will result in greatly enhanced spontaneous extension of the resulting filaments. The net change in length observed as the result of the treatment of the oriented filaments is either a net extension in length; or, if the net change in length is a shrinkage, the degree of growth of the shrunk filaments 'With respect to the degree of shrinkage of the oriented filaments is enhanced as contrasted with the corresponding oriented filament which "ice have not been treated in accordance with the novel process. 4

Briefly stated, the present invention comprises the process of extruding a molten linear terephthalate polyester to form filaments or other structures having a birefringence in the range of about 0.01 to about 0.06, and subsequently heating said structures at a temperature of at least about 65 C. while the structure is held at constant length.

The following examples will serve to illustrate the preparation of spontaneously extensile filaments without employing a shrinkage step, by means of the constant length heating process of the invention. These examples, as Well as other examples included hereinbelow, are not intended to be limitative, however.

EXAMPLE 1 Polyethylene terepht-halate having an intrinsic viscosity of 0.58 is spun at 295 C. through a spinneret having 34 orifices, each 0.009 inch in diameter. The extruded filaments are cooled in the conventional manner with a crossfiow current of air and are wound up as a yarn at a speed of 2800 yards per minute. The yarn is found to have a denier as spun of and a birefringence of 0.029. When a sample of the spun yarn is dipped in water at C. for an exposure time approximating 0.1 second, it shrinks 58%. The yarn is then immersed again in water at 100 0., this time for a period of 5 minutes. Additional shrinkage of the yarn is observed, amounting to 20% shrinkage based on the length of the yarn after the first shrinkage treatment.

Another length of the spun yarn, wrapped around a metal frame in such a manner that shrinkage is prevented, is immersed in 78 C. Water for 10 minutes. When the treated yarn is immersed in water at 100 C. for an exposure time apprixirnating 0.1 second, no'change in length is observed; however, upon leaving the sample in the boiling water for an additional five minutes, an extension in length is observed amounting to 2.2% of the length of the yarn after the constant length treatment.

EXAMPLE 2 Polyethylene terephth-alate yarn is spun as in Example 1, except that a wind-up speed of 2500 yards per minute is employed and the rate of spinning is such that the asspun denier is 50. The as-spun birefringence of the yarn is 0.034. When a sample of the spun yarn is dipped in water at 100 C. for an exposure time approximating 0.1 second, it shrinks 55%. Upon further immersion of the yarn for an additional five minutes in the boiling water, additional shrinkage is observed amounting to 23%, based on the length of the yarn after the initial shrinkage step.

Another sample of the spun yarn is passed from a feed roll to a pair of rolls 6 inches in diameter, rotated in opposite directions and maintained at a temperature of C., the yarn Ibeing wound in a figure 8 wrap having a total length of 25 inches per wrap. A total of 4 wraps are taken around the heated rolls, after which the yarn is wound up on a suitable package. The feed roll, heated rolls, and yarn package are all rotated at Patented May 28, 1963.

uniform peripheral speeds of 383 yards per minute in order to maintain the yarn at constant length. When a sample of the heat-treated yarn is dipped in 100 C. Water for an exposure time approximating 0.1 second, no change in the length of the yarn is observed; however, after leaving the yarn in the boiling water for an additional five minutes, it is observed that the yarn has grown 3.1%, based on the length of the original spun yarn. H

In accordance with the invention it has been found that the nature of the filament or other shaped structure employed as a starting material is critical. The first stage of the process of the invention therefore comprises extruding a molten linear terephthalate polyester from an orifice to form a structure having a longitudinal axis, cooling the extruded structure until it is solidified, and pulling the structure away from the extrusion orifice to orient it Within the birefringence range of 0.01 to 0.06, measured after the structure has solidified and cooled. The speed required to orient the structure within this birefringence range is somewhat dependent upon the diameter of the structure, the intrinsic viscosity of the polymer of which the structure is composed, and certain other factors. In general, however, the speed at which the structure is pulled away from the extrusion orifice should be within the range of about 1500 to about 4000 yards per minute, measured after the structure has completely solidified and cooled, when no more reduction in the diameter of the structure is observed. In the case of filaments and, yarns, this speedis designated as the spinning speed. The speed of the filaments at the first point at which they are contacted by a forwarding apparatus, or the speed of the wind-up if there is no intervening forwarding apparatus, provides a direct measure of this speed. The volume of polymer extruded per unit time is adjusted to provide filaments of the desired denier at the spinning speed employed.

With filaments having an as-spun birefringence of less than about 0.01, corresponding to a spinning speed of approximately 1500 yards per minute depending on the denier and other factors as discussed above, the constant length heat treatment of the present invention appears to have little or no etficacy in producing spontaneously extensible filaments, with or without a subsequent shrinkage treatment. Similarly, filaments having an as-spun birefringence of more than about 0.06, corresponding to a spinning speed of approximately 4000 yards per minute, appear to be. unsuited for use as starting materials in the process of the present invention. It should be noted that the process of the present invention is adapted for use only with filaments spun within this approximate range of spinning speeds, and not subsequently oriented further by drawing.

In its most preferred form, the constant length heat treatment of the present invention is coupled with a subsequent shrinkage step. With the addition of the shrinkage step, the spontaneous extensibility of the product can be maximized; while the use of the constant length heat treatment prior to the shrinkage step minimizes the amount of shrinkage required in the process to obtain a spontaneously extensible product. In any event, shrinkages are uniformly lower when the constant length heat treatment is employed than when the corresponding oriented filaments are processed to spontaneously extensible products in the absence of the constant length heat treatment. In the case of filaments having an as-spun birefringence of less than about 0.04, corresponding to spinning speeds below about 3000 yards per minute, the tendency towards shrinkage is so great that spontaneously extensible filaments are usually not obtainable without employing the constant length heat treatment of the invention.

The preferred shrinkage conditions which are used in conjunction with the constantlength heat treatment are analogous to those used by Kitson and Reese in their process for producing spontaneously extensible structures, disclosed and claimed in United States Patent 2,952,879. The starting material filament or other structure, prepared as described herein via the constant length heat treatment of the invention, is heated by passing it through a zone maintained at a temperature of at least about 90 C., the structure undergoing shrinkage while passing through said zone and being passed out of the said zone and cooled before the crystallinity of the. structure reaches the maximum crystallinity level achievable in said structure in said zone. The order of maximum exposure time is preferably the same as disclosed by Kitson and Reese in their Table I; in general, very short exposure times are the most desirable (e.g., 0.1 second in boiling water). In the present instance, however, a minimum shrinkage of 20% (as disclosed by Kitson and Reese) is definitely not required, and in some cases a shrinkage of only a fraction of 1% is encountered. Shrinkages of this low order of magnitude are useful, of course, only if this length change will be considered useful when added to the spontaneous extensibility of the product. For example, if the shrinkage encountered in a filament after constant length heat treatment is only 0.2% and the spontaneous extensibility of the resulting product is 3.0%, the filament resulting directly from the constant length heat treatment can be employed as a product having a spontaneous extensibility of 2.8%, omitting the shrinkage step entirely. On the other hand the maximum shrinkage values encountered when employing a shrinkage step subsequent to a constant length heat treatment step are on the order of 50% or slightly higher, rather than the 70% maximum shrinkage values encountered in accordance with the process described and claimed by Kitson and Reese.

to be unaffected. The maximum temperature at which the.

process may be carried out is less definite, but temperatures of 200 C. or even higher can be employed. At temperatures up to about 80 C. it appears that the treatment time may be indefinitely long (up to at least about 24 hours). Above about 80 C. a maximum time limit begins to become apparent, and as the temperature increases the length of the maximum treatment time drops very sharply. At 90 C. the maximum desirable exposure time has already dropped to about 40 seconds, and above this temperature the time continues to drop sharply. Obviously, the time becomes very short as the softening point of the polyester is reached, and at this temperature it is probable that the temperature imposed during treatment is actually higher than the temperature within the polyester. The character of the treatment medium has an effect, shorter times being required in water than under dry conditions to achieve similar results. Bearing in mind that the maximum treatment time is affected by numerous variables, Table I below lists, for a series of representative treatment temperatures, the order of maximum exposure time which has been found to be desirable for carrying out the constant length heat treating process of the present invention. Although the listed maximum exposure times represent regions of gradual rather than abrupt change, exposure times substantially greater than the order of maximum exposure time indicated in Table I, i.e., more than about twice the indicated time, usually do not result in useful spontaneously extensible products, with or without a subsequent shrinkage step.

Table 1 Temperature of constant Order of maximum length heat treatment: exposure time Up to about 80 C hours 24 90 C. seconds 40 100 C. do 125 C do 1.5 150 C do 0.75

There is no definite minimum exposure time for the constant length heat treatment step. As soon as the step is begun, the degree of shrinkage of the spun filament begins to decrease. After an interval optimum properties are achieved, the duration of the interval depending on the particular star-ting material as well as the conditions being employed; subsequently, the degree of extensibility or potential extensibility falls off as the duration of treatment is lengthened, until little or no potential remains above the maximum exposure time.

The expression structures having a longitudinal axis is used herein in the sense defined by Kitson and Reese to denote shaped articles of polymers in which at least one dimension of the structure is relatively quite large and at least one dimension of the structure is relatively quite small. The expression therefore comprehends ribbons and films as well as filaments and fibers. The structures are normally extruded from an orifice having a fixed cross section at a uniform rate of extrusion, and accordingly have a constant cross section. The structures may have a round cross section, an odd-shaped cross section such as a cruciform or Y-shaped section, or an elongated cross section as in a ribbon or film. Of course, a plurality of filaments may be spun together to form a yarn or the like; and at some point during the processing, usually after the process of the present invention has been carried out, the filaments may be cut to staple fibers, if desired.

The term spontaneous and irreversible extension, as used herein, refers to the amount of increase in length of the structure, i.e., growth along its longitudinal axis, when the structure is heated. The term spontaneous and irreversible extensibility is also employed, especially with reference to the potential or capability existing or remaining in the filament or other structure for growth when the structure is heated. An important characteristic of the increase in length of the structure along its longitudinal axis is that the increase in length is irreversible; that is, the product does not return to its original length when it is cooled or dried. Another important characteristic of the phenomenon of growth observed in the extensible filaments is that the increase in length is spontaneous and occurs without applying tension to the ends of the filaments.

The terms intrinsic viscosity, birefringence, crystallinity and maximum crystallinity level achievable and linear terephthalate polyester have been described or defined by Kitson and Reese in United States Patent 2,952,879 and are employed herein in the same sense.

The following examples are illustrative of the use of the constant length heat treatment process of the present invention, coupled with the preferred subsequent shrinkage step as described above. In the examples, shrinkage values are represented by the letter S and are calculated in accordance with the following formula:

as c:

: original length shrunk length X 100 S original length 6 cordance with the formula:

final lengthshrunk length shrunk length As above, the term shrunk length refers to the length of the filament just after shrinkage, while the term final length refers to the length of the filament after the step of spontaneous and irreversible extension has taken place. A positive value for the quantity E thus refers to the percentage increase in length. correspondingly, a negative value for the quantity E represents a decrease in length, or in other words additional observed shrinkage as a percentage based on the length after the first shrinkage step.

Net extension is represented by the abbreviation NE and is calculated in accordance with the formula:

final lengthoriginal length original length X% As above, the term original length refers to the length of the filament just prior to the shrinkage step, and the term final length refers to the length of the filament after the step of spontaneous and irreversible extension. The net extension is thus a measure of the amount of growth which can be achieved in the filament resulting from the constant length heat treatment process, regardless of any intervening shrinkage step.

EXAMPLE 3 In a series of experiments, polyethylene terephthalate having an intrinsic viscosity of 0.57 is spun at 295 C. through a spinneret having 34 orifices, each 0.009 inch in diameter, the extruded filaments being cooled in the conventional manner with a cross-flow current of air. Various windup speeds are employed, as indicated in the results recorded in Table II below, the rate of polymer extrusion being adjusted such that the spun denier is 70 in each instance. The birefringence of the spun yarns is shown in the table. S (shrinkage) values obtained by immersing each of these spun yarns in Water at 100 C, for an exposure time approximating 0.1 second are recorded, together with E (extension) values obtained by subsequently immersing the yarns in 100 C. Water again for 5 minutes. Most of the E values are negative, representing additional shrinkage; in the last two experiments recorded in the table, positive E values are obtained, but the spun yarn NE (net extension) values are overwhelmingly negative in each instance.

Table II CONSTANT LENGTH HEAT TREATMENT IN 78 0. WATER (10 MINUTES) Properties of yarn Spun Properties of spun heat-treated at Spmmng yarn yarn, percent constant length, Item speed, birepercent y.p.m. fringence S E NE S E NE Samples of each of the spun yarns are then Wrapped around a metal frame in such a manner as to hold the yarns at constant length, after which they are immersed in 78 C. water for 10 minutes in each instance. S values obtained by dipping the heat-treated yarns in water at 1000 C. for an exposure time approximating 0.1 second in each instance are recorded, together with E values obtained by subsequently immersing the shrunk yarns again in 1000 C. water for 5 minutes. The corresponding NE values are also recorded for each sample.

EXAMPLE 4 Polyethylene terephthalate having an intrinsic viscosity of 0.59 is spun at 295 C. through a spinneret having 34 orifices, each 0.009 inch in diameter, and the yarn is wound up after conventional cross-flow cooling at a speed of 3000 yards per minute. The resulting 34-fi1ament yarn is found to have a denier as spun of 50 and a birefringence of 0.039. \Vhen a sample of the spun yarn is dipped in water at 100 C. for an exposure time approximating 0.1 second, an S value of 55% is obtained. The yarn is then immersed again in water at 100 C., this time for a period of minutes, resulting in an E value of 1.5%.

Another length of the spun yarn, wrapped around a metal frame in such a manner that shrinkage is prevented, is immersed in 65 C. water for 5 minutes. When the treated yarn is dipped in water at 100 C. for an exposure time approximating 0.1 second, an S value of 26% is observed. The yarn is then immersed in 100 C. water for 5 minutes, resulting in an E value of 17%. In a similar experiment, wherein the constant length heat treatment is carried out in 100 C. water for 0.1 second rather than 65 C. water for 5 minutes, the corresponding S and E values are 6% and 4%, respectively. In another experiment, constant length heat treatment in 100 C. water for 1 second is carried out on the same spun yarn, resulting in S and E values of 1% and 1%, respectively.

A series of samples of the same spun yarns are similarly subjected to constant length heat treatment in 90 C. water for various periods of time, after which each sample is subjected to shrinkage in 100 C. water for 0.1 second and then re-immersed in 100 C. water for 5 EXAMPLE 5 In a series of experiments, polyethylene terephthalate having an intrinsic viscosity of 0.59 is spun at 295 C. through a spinneret having 34 orifices, each 0.009 inch in diameter, with conventional cross-flow cooling. Various windup speeds are employed, as indicated in the results recorded in Table IV below, the rate of polymer extrusion being adjusted such that the spun denier is 50 in each instance. The birefringence of the spun yarns is shown in the table. S values obtained by immersing each of these untreated spun yarns in water at 100 C. for an exposure time approximating 0.1 second are recorded, together with E values obtained by subsequently immersing the yarn in 100 C. water again for 5 minutes.

Samples of each of the spun yarns are passed from a feed roll rotated at a peripheral speed of 383 yards per minute to a pair of heated rolls 6 inches in diameter rotated in opposite directions at the same peripheral speed as the feed roll in order to maintain the yarn at constant length. The yarn is wound around the two rolls in a figure Swrap having a total length of 25 inches per wrap. The number of wraps taken around the heated roll is varied to provide the desired duration of yarn heat treatment, the exposure time being 0.11 second per wrap. The temperature of the roll and the duration of heat treatment for each experiment are recorded in the table. Upon leaving the heated roll, the yarn is wound up on a suitable package also rotated at a peripheral speed of 383 yards per minute. Samples of each of the heat treated yarns are then immersed in 100 C. water, first for an exposure time approximating only 0.1 second to obtain an S value and 5 then again for an addltronal 5 minutes to obtain the E and NE values.

Table IV CONSTANT LENGTH HEAT TREATMENT ON HEATED ROLLS AT 383 Y.P.M.

Spmn- Spun Dura- Properties of treated ing yarn Roll tion of yarn, percent Item speed, bircfrintemp, treaty.p.m. gence C. ment,

sec. S E NE 1 0. 034 Untreated spun 55 23 65. 4

yarn O. 034 100 O. 66 32 16 21. 1 0. 034 105 0. 11 50 9 45. 5 0. 034 125 0. 11 11 6 5. 7 0. 034 125 0. 33 1. 0 3. 5 2. 4 6 0. 034 125 0. 66 0. 0 2. 9 2. 9 0.052 Untreated spun 34 16 23. 4

yarn 0. 052 113 I 0. 66 1. 3 2. 1 0. 7 0.052 125 0. 11 2. 3 1. 1 1. 2 0.026 Untreated spun 49 40 69. 4

yarn 11 2, 000 0. 026 130 1. 54 4. 3 3. 6 0. 9 12 w 2, 000 O. 026 140 0. 88 1. 2 2. 4 1. 2

Item 6 of Table IV exhibited a tenacity of 2.0 g.p.d., an elongation of a modulus of 14 g.p.d., and a yield point of 0.5 g.p.d.

EXAMPLE 6 A sample of the spun yarn of item 1 in Table IV is passed from a feed roll through a hollow needle leading into a nozzle having a throat diameter of 0.062 inch and a 7 flared exit passage and thence to a suitable wind-up package. Air is maintained at C. and 9 p.s.i. pressure on the entrance side of the nozzle so that a jet of hot air is caused to flow through the nozzle in the same direction as the yarn is passed through the nozzle. The tip of the hollow needle from Which the yarn is delivered is located within the throat of the nozzle and the effective distance through which the yarn is heated is 1.35 inches. The yarn is passed from the feed roll into the nozzle at 33.3 yards per minute and wound up at the same speed (i.e., constant length heat treatment), corresponding to an exposure time of 0.067 second. A sample of the heattreated yarn is immersed in 100 C. water, first for an exposure time approximating only 0.1 second whereupon an S value of 38% is obtained and then again for an additional 5 minutes, whereupon an E value of 15% is obtained.

The experiment is repeated, using an air jet at C. for an exposure time of 0.067 second (33.3 y.p.m. feed roll speed and wind-up speed). An S value of 25% and and E value of 6% are obtained. The feed roll and windup speeds are then increased to 133.3 yards per minute, reducing the exposure time to 0.017 second but maintaining the jet temperature at 150 C. An S value of 49% and an E value of 10% are obtained.

EXAMPLE 7 e Yam Spmnmg speed, y.p.m. birefringence S, percent E, percent 9 10 Samples of each of the spun yarns are passed from a We claim: feed roll to a pair of heated rolls 6 inches in diameter 1. The process of preparing a structure having a potenrotated in opposite directions at the same peripheral speed tial for spontaneously and irreversibly extending in length as the feed roll. The yarn is wound around the two rolls upon heating which comprises forming the structure by in a figure 8 wrap having a total length of 25 inches per extruding a molten linear terephthalate polyester through wrap. The number of wraps taken around the heated an orifice, cooling the extruded structure until it solidiroll is varied to provide the desired duration of yarn heat fies, pulling the structure away from the extrusion orifice treatment, at least half a wrap being taken in each instance .to orient it within the birefringence range of 0.01 to 0.06, to ensure constant length heat treatment of the yarn as it nd subsequently heating said tructure at a temperature contacts the heated rells- The temperature of the heated above about 65 C. while the structure is held at constant rolls, peripheral speed of the heated rolls, and duration length of heat treatment at constant length for each experiment The process f claim 1 in which the potential Of are recorded In Table leavmg the heated roll the structure for spontaneous and irreversible extensibility the yam Wound up on a Smtable Package rotated at a is increased by subjecting the said structure to a subselower peripheral speed than that of the heated roll to quent Shrinkage Step 5232i? 3.35533322121 2353 to s? l r g'ilii iii eifi The of claim in which the Shrinkage Step yarn. In some instances the yarn, after leaving the heated 1s carneFl i i saldf i i g g roll, is passed through a hollow needle leading into a Zone i mm a a empera We 0 i We a on and passing the structure out of the sa1d zone and cooling nozzle having a throat diameter of 0.062 inch, a throat length of 1 inch and a 0 flared exit passage of 135 inch it before the crystalllmty of the structure reaches the length b f reaching the windqlp, to f ilit t the Shrink maximum crystallinity level achievable in said structure age step. Air is maintained at 180 C. and p.s.i. presm sa1d Zone; the Speed of P g the Struetllre ut f sure on the entrance side of the nozzle, so that a jet of the Zone being lower than the Speed Of feeding the Slime hot air is caused to flow through the nozzle in the same 25 lure into the Zone to effect Shrinkagedirection as the yarn is passed through the nozzle. 4. The process of claim 1, in which the stru t i Table V CONTINUOUS CONSTANT LENGTH HEAT TREATMENT FOLLOWED BY SHRINKAGE Spinning Hot roll Hot roll Duration of Wind-up Under)drivc Item speed, temp, speed, constant speed, (S E,

y.p.m. 0. y.p.m. n y.p.m. percent employed percent heating, sec.

The presence or absence of the et is noted m the table. heated at constant length for a period not greater than Samples of these yarns, which have been subjected both that indicated in Table I for the temperature of the to a constant length heat treatment and a subsequent constant length heat treatment.

shrinkage step, are immersed in 100 C. water for 5 5. The process of claim 1, in which said polyester is minutes. Spontaneous extension of the yarn is observed, polyethylene terephthalate.

the E values being noted in the table. 6. The process of claim 1, in which the said structure Yarn from item 4 of Table V has a tenacity of 1.4 is in the form of a fiber.

g.p.d., an elongation of 220%, a modulus of 15 g.p.d., 7. The process of claim 1, in which the structure is and a yield point of 0.5 g.p.d. The yarn is plied with a pulled away from the orifice at a rate of from about 1500 standard, commercially available 70 denier, 34 filament to 4000 yds./minute measured after the structure has yarn having a shrinkage of 12% when immersed in 100 s lidified,

C. water for 5 minutes. When woven as the filling in a standard 2 x 1 basket construction, the yarn yields a References Cited in the file of this patent fabric which bulks when treated in 100 C. water for 5 minutes and exhibits a warm, soft hand with good cover. UNITED STATES PATENTS It will be apparent that many widely different embodi- 2,604,689 Hebelel July 29, 1952 ments of this invention may be made without departing 2,734,794 Carlton Feb. 14, 1956 from the spirit and scope thereof, and therefore it is not 2,917,779 KUrZke 6t 81 Dec. 22, 1959 intended to be limited except as indicated in the appended 2,987,373 Bezerner et al. June 6, 1961 claims. 3,030,173 Kurzke et al. Apr. 17, 1962 

1. THE PROCESS OF PREPARING A STRUCTURE HAVING A POTENTIAL FOR SPONTANEOUSLY AND IRREVERSIBLY EXTENDING IN LENGTH UPON HEATING WHICH COMPRISES FORMING THE STRUCTURE BY EXTENDING A MOLTEN LINEAR TEREPHTHALATE POLYESTER THROUGH AN ORIFICE, COOLING THE EXTRUDED STRUCTURE UNTIL IT SOLIDFIES, PULLING THE STRUCTURE AWAY FROM THE EXTRUSION ORIFICE TO ORIENT IT WITHIN THE BIREFRINGENCE RANGE OF 0.01 TO 0.06, AND SUBSEQUENTLY HEATING SAID STRUCTURE AT A TEMPERATURE ABOVE ABOUT 65*C. WHILE THE STRUCTURE IS HELD AT CONSTANT LENGTH. 